U.S. Pat. No. 6,315,812 issued to Fleming, et al. Nov. 13, 2001, describes an oxidative pressure leaching process for recovery of metals using halide ions. The method disclosed therein comprises the steps of subjecting the host material to an oxidative pressure leach process, in the presence of a halide ion constituent which is reactive with the precious metal, at a temperature sufficient to cause at least a portion of the precious metal to be extracted by a leach solution; and recovering the precious metal from the leach solution.
In accordance with the process described in U.S. Pat. No. 6,315,812, one or a number of the precious metals and base metals may be leached from the host material in a single oxidative pressure leach process, whose leach solution may be treated to recover the individual precious and base metals, as desired, and the reaction can occur with a relatively low concentration of halide ion in the leach solution, provided the selected halide ion is sufficiently reactive with the precious metals being recovered. The method involves the following steps: i) subjecting the host material to an oxidative pressure leach process, in the presence of a leach solution bearing halide ion constituent which is reactive with the precious metal, and at a temperature sufficient to cause at least a portion of the precious metal to be extracted by the leach solution; and ii) recovering the precious metal from the leach solution. The host material may be a sulphide ore or ore concentrate. The sulphide ore or ore concentrate may contain economic quantities of base metals, for example copper, nickel, cobalt or zinc. During pressure leaching in the presence of oxygen, the sulphide component of the host material is oxidized to form sulphuric acid in solution. In which case, the process takes place in an acidic environment ranging from 20 to 100 g/L sulphuric acid. In effect, the acid is produced during the sulphide oxidation reaction. The base metals are co-leached or solubilised in the same acid solution. Accordingly, the leaching of precious metals and base metals from the host material occurs in the presence of sulphuric acid, and in alternative embodiments may take place at a temperature of at least 200 degrees Celsius, at temperatures below 200 degrees Celsius and at temperatures as high as economically achievable. The temperature selected for the pressure leach reaction depends on the mineralogical characteristics and particle size of the feed material, the concentration of precious metals in the feed material, the acid strength, the oxygen partial pressure and the residence time in the pressure leaching vessel. When the halide ion is a chloride, the chloride salt may be selected from sodium chloride, calcium chloride or potassium chloride. Hydrochloric acid may also be used as a source of chloride. In selected embodiments, the salt is sodium chloride and is provided at a concentration ranging from about 1 g/L to about 100 g/L, or from 2 to 20 g/L, or from 5 to 15 g/L, or from 8 to 12 g/L. In one embodiment, the chloride salt is sodium chloride at a concentration of about 10 g/L in solution. The optimum concentration of the chloride ion may be higher with increasing concentration of the precious metal being recovered.
In some embodiments of the Fleming et al. process, the oxidative pressure leach process takes place in the presence of a gaseous oxidant. The gaseous oxidant may be oxygen gas, and the oxygen gas may be injected into the vessel at an oxygen partial pressure of between 1 and 500 psig, or between 10 and 200 psig, or between 50 and 100 psig.
In the Fleming et al. process, once the precious metals are in solution, and the solution is separated from the leached solid residue, a number of approaches may be selected to recover the precious metal constituent first, such as by the use of activated carbon adsorption or ion exchange resin adsorption, or reduction/precipitation with a mixture of sulphur dioxide and sodium hydrosulfide, as well as copper cementation or cementation on a metal sulphide such as copper sulphide. In the case of adsorption on activated carbon or ion exchange resins, the precious metals constituent may be recovered directly, if desired, from the acidic pulp phase prior to solid liquid separation, which may be advantageous in some circumstances.
In some embodiments of the Fleming et al. process, the autoclave residence time for leaching of metals may be relatively long, such as two or more hours, to achieve acceptable levels of metal recovery.